Multi-WAN Outbound NAT Issues

I recently switched my ISP to one where each individual Static IP needed a hardware MAC to associate with it. Because I have 5 Static IPs and 1 Dynamic, I had to add 5 NICs to my pfSense router. All this is done and my router is working just fine EXCEPT for Outbound NAT. Everything was working fine on my old setup with Virtual IP addresses but it doesn't seem to work the same with multiple WAN interfaces. Here's the deal:

Normally all traffic is routed through WAN but I need all the traffic from my mailserver server (192.168.1.181) to be routed through MAIL Interface. See my screenshot attached, should this work?

I had to add two LAN Firewall Rules. See the rules in the screenshot attached. Note that I had to select the non-default gateway that corresponded to the interface that I was trying to force the traffic out on. I have no idea where I would have found this in the documentation or even if I'm actually doing this correctly, but it seems to work for me.